USG basics instruments AND PRINCIPLES.pptx

USG principle and basics

1. ULTRASOUND PRINCIPLES AND INSTRUMENTATION

2. Ultrasonography Definition Imaging technique that uses high frequency sound waves to produce images of internal structures of the body. R ecords the reflections (echoes) of ultrasonic waves. Diagnostic medical ultrasound – - use of acoustic properties of bodies - produce image from moving & stationary tissue

3. PROPERTIES OF ULTRASOUND Ultrasound travels in longitudinal direction, but sound waves can’t travel through the vacuum. Sound travels slowest in gases, faster in liquids and most rapidly in solids Strength of reflected sound  Density difference of the medium (acoustic impedance). Acoustic impedenec (Z)=Density(D) x Propagation velocity(V)

4. Production of USG waves An assembly meant for production and detection of ultrasound. It converts an electric signal into ultrasonic energy that can be transmitted into tissues and ultrasonic energy reflected back from the tissues into an electrical signal. The functional component being the piezoelectric crystal having electromechanical properties TRANSDUCER

5. Piezoelectric crystal The piezoelectric element is comprised of aligned molecular dipoles. A. Under the influence of mechanical pressure from an adjacent medium (e.g., an ultrasound echo), the element thickness contracts (at the peak pressure amplitude), achieves equilibrium (with no pressure), or expands (at the peak rarefactional pressure) causing realignment of the electrical dipoles to produce positive and negative surface charge. Surface electrodes (not shown) measure the charge variation (voltage) as a function of time B. An external voltage source applied to the element surfaces causes compression or expansion from equilibrium by realignment of the dipoles in response to the electrical attraction or repulsion force. Made of lead- zirconate - titanate (PZT) Molecular dipoles structure Principle of Curie temperature (328°C to 365°C)

6. PIEZOELECTRIC CRYSTAL Application of voltage across crystal in pulse ↓ Change in alignment within crystal ↓ Production of ultrasonic waves ↓ Ultrasonic waves travel through tissue layers and are echoed from various tissue interfaces ↓ Return echoes strike piezoelectric crystal causing physical compression within ↓ Change in orientation of dipoles-potential difference across electrodes

7. Types of transducers

8. Linear Array transducer Individual elements or groups of elements are fired in sequence. This generates a series of parallel ultrasound beams, each perpendicular to the transducer face. These beams generate line of sight that combine and form final image. Produce rectangular image format Used for evaluation of superficial structures- small parts, vascular, thyroid, breast. High frequency- 7.5-15 MHz

9. CONVEX (CURVED) ARRAY Variants of linear array design. E lements are used in curved position rather than straight. It produces image that combines a relatively large surface field of view with sector display format. Operation is similar to linear array. Used For evaluation of deeper strcture - general abdominal, ostetrics and transabdominal pelvic scan. Low frequency- 2.25- 5 MHz 10

10. PHASED ARRAY Construction similar to linear array. All transducer elements are activated nearly simultaneously Focusing is achieved by delay time By using time delay generator the ultrasound beam can be steered and focused electronically without physically moving the transducer on the patient. Delay changes the direction of the pulse produced. Sophisticated detection algorithms synthesize the data to form the image.

11. Interactions of Ultrasound with Matter Reflection Absorption Attenuation Refraction Scatter Diffraction

12. Focus Knob

13. Depth Start with higher depth Decrease depth to areas of interest to 3/4 th depth of screen Leave small areas behind to know shadowing enhancement

14. TIME GAIN COMPENSATION TGC is a user adjustable amplification of the returning echo signals as a function of time Ideal TGC curve makes all equally reflective boundaries equal in signal amplitude regardless of the depth of the boundary

15. Gain Overall amplification of returning ultrasound signal received from all depths TGC Each slider for a range of depth Top : superficial Bottom : deep Buttons at center :average gain

16. Modes of Ultrasound A mode : Amplitude of signal processed based on A line Initially used in mass effect of brain tumors Currently in ophthalmology. Spikes replaced by spots. The brightness of the spot -the strength of the echo. B mode

17. M Mode S pots on the display produce a tracing of the motion of an echogenic object. The echo data from a single US beam passing through moving tissues is acquired and displayed as a function of time